The present invention relates to a laser beam scanning apparatus in which a laser beam of which the luminance is modulated on the basis of a picture signal is deflected in a main scanning direction so as to illuminate on a substance to be illuminated, and, for example, may be used as an exposure means for a photosensitive member which is arranged in an image forming apparatus such as a laser printer.
An image forming apparatus such as a printer or a display unit is provided with a scanning apparatus for modulating the luminance of a light on the basis of a specified signal and illuminating the modulated light on a substance to be illuminated such as sensitive member, a photographic film or a screen.
In such a scanning apparatus, a laser beam source capable of obtaining a very small spot serving as a light source is employed, and then in order to prevent a laser beam emitted from the laser beam source from accidentally inserting into eyes of an operator, it is required to cover the apparatus with a light-intercepting cover under the safety standard
In the apparatus, as a deflecting means, there have been known means for rotating a polygonal mirror, means for oscillating a galvano mirror, means for rotating a hologram, etc. In the means, the first means for rotating a polygonal mirror is best generalized because of simple construction thereof as compared with the construction of the other means.
Since when the laser beam is deflected by a polygonal mirror, the scanning speed is in proposition to the number of rotation of the polygonal mirror, it is preferred to increase the rotational speed of the polygonal mirror in order to scan at high speed. However, this solution has some limitation such as the durability of a motor for rotating the mirror and the material of the mirror. Similarly, the other means have various kinds of problems to be solved for high speed operation in the mechanisms thereof, including a transfer time for transferring a picture data.
Therefore, specially, in a high-speed printer, plural laser beams modulated on the basis of different signals are employed, and those beams are simultaneously deflected to simultaneously scan plural parts of a substrate to be illuminated, resulting in achievement of the scanning operation at substantial several times higher speed than that of a normal speed printer.
On the other hand, in a scanning apparatus provided in a multi-color printer or a two-color printer which has a black and a white toners, more than two laser beams modulated on the basis of signals corresponding to individual color images are illuminated on different parts of a substance to be illuminated or substances to be illuminated to form multi-color images.
In the scanning apparatus utilizing such plural laser beams, in order to become small in construction and cheaper in manufacturing cost, there has been proposed an apparatus in which each laser beam is deflected by one deflecting means.
In the apparatus in which the one deflecting means is used in common as described above, there have been proposed an apparatus in which the incident angle of each laser beam incident on the means is different from each other, and an apparatus in which each laser beam is incident on the means while spacing each other. The former has bad constant velocity linearity of the scanning locus on a substance to be illuminated, while the latter is required to provide a large deflecting means having a big plane of incidence and has bad constant velocity linearity of the scanning locus on a substance to be illuminated.
Therefore, the conventional apparatus is provided with plural laser beams having individual characteristics such as wavelengths or polarizing directions. Then, these beams are composited to form one laser beam and then the laser beam is deflected in a main scanning direction. After that, the beam is split into plural laser beams, which are before composition, on the basis of the individual characteristic which each laser beam before composition has. Each of the split laser beams illuminates on a different part of a substance to be illuminated.
For example, in Japanese Laid-open Patent Publication No. 60-32019, there is disclosed an apparatus in which, in order to perform the high-speed scanning operation, two laser beams having different polarizing directions are composed and then deflected by a polygonal mirror, and the deflected laser beams are split by a polarizing beam splitter. In another Japanese Laid-open Patent Publication No. 60-201319, there is disclosed a scanning apparatus suitable for a color printer in which three laser beams with different wavelengths from each other are composed and split.
Generally, a beam splitter, by which after one laser beam composed plural laser beams is deflected, the laser beam is split on the basis of the characteristic of each laser beam before composition, employs a dichroic mirror with a partial reflective film for transmitting a laser beam with the specified characteristic and reflecting a laser beam with the other specified characteristic.
The dichroic mirror is so constructed that the surface of a transmitting material such as an optical glass is covered with a partial reflective film composed of a dielectric multilayer film or a metal-dielectric multilayer film. The mirror has the selectivity with respect to the wavelength or the polarized direction of a laser beam.
However, since the use of the partial reflective film with the excellent selectivity do not allow a laser beam to split completely, the transmitting beam transmitted the film and the reflective beam reflected by the mirror include the other laser beam with the different characteristic, respectively.
Therefore, when each of the split laser beam illuminates on the different part of a substance to be illuminated to form an image, it causes such a problem, called as a "ghost", that the image to be formed on the other part is unnecessarily overlapped on a desired image.